Apparatus including initial electrode charge maintaining means for measuring the concentration of an electrolyte

ABSTRACT

A device for measuring the concentration of an electrolyte utilizing measuring electrodes and a current generating means supplying the electrodes with a small current for maintaining them in their initial charge state and voltage measuring means connected to the electrodes for indicating the concentration of the electrolyte.

United States Patent Pitsch et al. 1 Apr. 25, 1972 54 APPARATUSINCLUDING INITIAL 51 Int. Cl. ..G0ln 27/42 ELECTRODE CHARGE MAINTAINING[58] Field of Search ....324/29, 29.5, 20, 94; 204/195, MEANS FORMEASURING THE 204/ 1 T CONCENTRATION OF AN [56] mums Cm ELECTROLYTEUNITED STATES PATENTS [72] Inventors: Johan Ludwig Pitsch, Nanterre;Gerard Ch i ig y Sur Orge, both of 2,832,734 Eckfeldt France 3,262,0517/1966 Payne ..324/29 3,275,534 9/1966 Cannon et al.. .324/29 X [73]Assignees: Compagnie Generale DElectricite; Com- 2,069,562 2/ 1937Schnorf ..204/195 pagnie Generale D'Automatlsme, Pan's, 2,912,36711/1959 Asendorf et al... ...204/ 195 X France 3,242,729 3/1966 Keller..204/195 X [22] Filed: 1969 Primary Examiner-Gerard R. Strecker [21]Appl, No.: 888,032 Attomey-Edwin E. Greigg Related US. Application Data[57] ABSTRACT [63] Continuation-impart of Ser. No. 558,510, June 17, Adevice for measuring the concentration of an electrolyte 1966,abandoned. utilizing measuring electrodes and a current generating meanssupplying the electrodes with a small current for maintaining [30]Foreign Application Priority Data them in their initial charge state andvoltage measuring means US. Cl ..324/29, 204/ 195 connected to theelectrodes for indicating the concentration of the electrolyte.

13 Claims, 13 Drawing Figures Patented April25,1972 3,659,193

5 Sheets-Sheet 1 Patented April 25, 1972 5 Sheets-Sheet 2 FIG. 2.

FIG.3

Patented April 25, 1972 3,659,193

5 She ets-Sheet s W I FIG. 5

Patented April 25, 1972 3,659,193

5 Sheets-Sheet 4 H FIG. 9

FIG. IO

Patented April 25, 1972 3,659,193

5 Sheets-Sheet 5 1 FIG. ll

APPARATUS INCLUDING INITIAL ELECTRODE CHARGE MAINTAINING MEANS FORMEASURING THE- CONCENTRATION OF AN ELECTROLYTE This application is acontinuation-in-part application of Ser. No. 558,510, filed June 17,1966, now abandoned.

The invention relates to the measurement or determination of theconcentration of an electrolyte.

The concentration of an electrolyte is sometimes measured by means oftwo charged electrodes immersed in the'electrolyte. In certain measuringconditions, the voltage between the two electrodes is a function ofconcentration and if this voltage is measured the concentration of theelectrolyte may be determined.

, Nevertheless, devices produced to date involve certain drawbacks.Specifically, lack of stability of the initial state of charge of theelectrodes as a function of time causes imprecise measurement and makesit necessary to recharge the electrodes from time to time.

Moreover, during these recharges, the electrodes acquire a voltage bypolarization which is no longer representative of the concentration ofthe electrolyte. The polarization phenomenon fades away slowly andduring that time the electrodes are unusable.

The time constant of the variation of the voltage as a function of thevariation of the concentration of the electrolyte for electrodespreviously used makes it difficult to measure accurately rapidvariations of concentration.

This invention serves to overcome the above-mentioned drawbacks byproviding a device for measuring the concentration of an electrolyte inan electro-chemical generator, comprising two measuring electrodesimmersed in the electrolyte, a voltage measuring apparatus connectedwith the terminals of said electrodes, and a means for supplying aquantity of electricity to each of the electrodes to enable them toretain their initial charge state. The polarization P is made negligibleby utilizing a weak current and continually supply the same as eitherdirect or stepped to the electrode terminals.

Accordingly, it is an object of the invention to provide a device formeasuring the concentration of an electrolyte having measuringelectrodes maintained at an optimum charge state, and wherein themeasurement is obtained with higher precision and reliability.

It is another object of this invention to provide a device for measuringthe concentration of an electrolyte which, when the charge circuits ofeach of the electrodes areseparate, it is possible to automaticallycompensate separately possible variations in the measuring conditions.

It is yet another object of this invention to provide a device formeasuring the concentration of an electrolyte in which the use ofelectrodes with low response time enables the rapid concentrationvariations to be detected and measured.

It is still another object of this invention to provide a device formeasuring the concentration of an electrolyte which does not cause anydeterioration or any influence upon other circuits or elements connectedto any electrode immersed in the same electrolyte.

According to one embodiment of the invention, the device for measuringthe concentration of an electrolyte comprises two measuring electrodesimmersed in the said electrolyte, electric charge generating means orcurrent source operatively connected to the said measuring electrodesand capable of supplying each of the electrodes during a predeterminedperiod with a quantity of electricity maintaining them in their initialcharge state, and a voltage measuring means connected to said measuringelectrodes, the indication of said voltage measuring means beingrepresentative of the concentration of the said electrolyte during thesaid predetermined period.

According to another embodiment, the device comprises charge generatingmeans including a current generator the terminals of which are connectedrespectively to the said measuring electrodes, said electrodes havingactive surfaces the ratio of the areas of which is equal to the inverseof the ratio of quantity of current per surface unit necessaryrespectively to each of the electrodes in order to maintain them attheir initial charge state.

In order that the invention may be more clearly understood severalembodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a diagram showing the variation of potential of each of theelectrodes as a function of the current:

FIG. 2 shows one embodiment of the invention incorporating a currentgenerator operative to supply current to the electrodes;

FIG. 3 shows another embodiment of the invention incorporating twocurrent generators operative to supply current to the two electrodesrespectively;

FIG. 4 graphically illustrates the currents supplied by the currentgenerators of the embodiment shown in FIG. 3 plotted against time; and

FIGS. 5 through 13 graphically illustrate other forms of currentvariation with respect to time for the currents of the two currentgenerators of FIG. 3, the corresponding variations in polarization ofthe electrolyte and the optimum measuring times.

Referring to FIG. 1 two curves are shown which represent the variationof the potential U of the positive and the negative electrodesrespectively measured with respect to a reference electrode, for examplea hydrogen electrode, for a given concentration of the electrolyte as afunction of the density of charging current i. The reference electrodesserves only to fix the origin of the potential scale since what isimportant is the relative variations of the potentials of the positiveand negative electrodes and not the individual value of thesepotentials. Each curve has a substantially horizontal portioncorresponding to the value of the density of the current which exactlybalances the loss of charge of the corresponding electrode, the mediumabscissae of these horizontal portions being indicated by i, for thepositive electrode and by i',, for the negative electrode.

If an error margin of A in the potential U of each electrode is allowed0 and i, then comes within the range defined by i,,,,, and i for thepositive electrode and i,,, and in for the negative electrode. It isnoted that, even if a relatively high error-margin is allowed, such asis shown, for example, in FIG. I, the two ranges do not overlap. Thisshows the necessity for choosing current densities of different valuesfor the two electrodes, so as to maintain a suitable charge state whileavoiding undesirable polarization.

Referring to FIG. 2 a circuit of simple construction is shown whichenables the obtention of a current density for one electrode differentfrom the current for the other electrode.

The electrolyte 6 is contained by a vessel 7. Two electrodes, a positiveelectrode 1 and a negative electrode 2, are immersed in the electrolyte6. The two electrodes are connected to current generator 3. A voltagemeasuring apparatus 13 is connected to the electrodes 1 and 2 by meansof conductors 11 and 12. Since the voltage across the electrodes is afunction of the concentration of the electrolyte between them, theconcentration of the electrolyte may thus be determined. In accordancewith the present invention, in order to obtain suitable chargedensities, the ratio of the area of the active surfaces of the twoelectrodes 1 and 2 is inversely proportional to the ratio of thecurrents which are supplied to them to keep them at their initial stateof charge.

Sometimes it is necessary, however, to have a surface ratio of about1/10, which necessitates the use of a prohibitively large electrode.Indeed, the dimensions and the positioning of the electrodes must bechosen so that the latter are immersed in a part of the electrolyte inwhich there is no appreciable concentration gradient, and so that thecomposition of the electrolyte where they are placed is representativeof the average concentration. With the above circuit, however, it isimpossible to vary the charge density of the electrodes independently ofeach other by varying the current fed thereto since they are bothenergized by the same current source.

FIG. 2 shows one arrangement for overcoming the aforenoted problem. Twoelectrodes using the same current employ the selection of two electrodeswhose lateral surfaces are in a 1:8 ratio, for example:

cm for the negative electrode 40 cm for the positive electrode.

The intensity of the current to be supplied to the electrodes isdependent upon the size of the exchange surface between the electrodesand the electrolyte. In order to maintain the electric charge state,namely, the initial state, it is necessary to provide a constantintensity per exchange surface unit, inv

other words, current density. This current density is not the same forboth electrodes. As in the case described in FIG. 2, however, the samecurrent can be made to circulate through two electrodes having differentsurface ratios which are equal to the inverse of those of the currentdensities. On the other hand, two currents having different values canbe made to circulate through two electrodes by means of an auxiliaryelectrode, such as shown in FIG. 3. In this embodiment, the twomeasuring electrodes need no longer maintain the surface ratios asrequired with the apparatus shown in FIG. 2.

FIG. 3 shows another embodiment of the invention. As with the circuitshown in FIG. 2, the positive electrode 1 and a negative electrode 2 areimmersed in an electrolyte 6 in a vesse] 7. In addition, however, anauxiliary electrode 16 is immersed in the electrolyte 6 and two currentgenerators 4 and 5 are provided. The current generators 4 and 5 areconnected respectively between the positive electrode 1 and theauxiliary electrode 16, and the negative electrode and the auxiliaryelectrode 16.

The generator 4 delivers a current i, to the electrode 1 through aconductor 8. This current then passes through the electrolyte 6, theauxiliary electrode 16 and the conductor 10. The generator 5 delivers acurrent i to the negative electrode 2 through the conductor 10. Thiscurrent then passes through the electrode 16, the electrolyte 6 and theconductor 9. The currents i and i are adjusted to supply the necessarycharges to each electrode, and this enables the use of electrodes of anyshape, and more particularly the use of electrodes having the samedimensions.

A voltage measuring apparatus 13 is connected to the two electrodes 1and 2 through the conductors 11 and 12. The concentration of theelectrolyte from the reading of the voltage measuring apparatus may thusbe determined as described above.

One form of the current variation with respect to time for the currentsof the two current generators shown in FIG. 3 is shown in FIG. 4. InFIG. 4 the current values i and i are constant with respect to time andare of differing magnitude. The current magnitudes are those necessaryto maintain the corresponding electrodes at the initial state of charge.

Three other forms of current variation are respectively shown in FIGS.through 7,8 through 9 and 10 through 13.

Referring to FIG. 5, FIG. 6 and FIG. 7 one of the generators supplies aconstant direct current whereas the other generator supplies a pulsatingcurrent. FIG. 5 shows the current i as a function of time t. It is notedthat the current i, of the positive electrode is constant and that thecurrent i of the negative electrode is of substantially square-waveform.

The peak value of the current i is higher than the constant value of thecorresponding current i; of FIG. 4. Its average value may even be higherthan the latter value.

FIG. 6 illustrates the variations of the polarization P of the twoelectrodes as a function of time t. The value of the polarization P ismeasured by the difference between the instantaneous voltage of anelectrode and the balance voltage obtained in the absence of current. Itis noted that the polarization of the positive electrode P is constantand that the polarization of the negative electrode P is ofsubstantially square-wave form. The shape and the amplitude ofthesesquare-wave variations cannot be defined with precision, but itshould be readily apparent that the moment when the polarization P, isstable and substantially constant corresponds to the rest periods of thecurrent i FIG. 7 represents the voltage signals measured between theelectrodes 1 and 2 of FIG. 3. The voltage 5,, of the positive electrode,is read continuously whereas the voltages S of the negative electrode,can only be validly read at moments t I corresponding to the restperiods of the current i and preferably at-the end of these restperiods, at which time the risk of the potential of the negativeelectrode being effected by polarization will be at a minimum.Measurements M M correspond the the times FIGS. 8 through 10 and 11through 13 show curves similar to those of FIG. 5 through 7 andcorrespond to the circuit of FIG. 3 when both current generators 4 and 5are capable of supplying pulsating currents.

Referring to FIG. 8 through 10, the currents supplied by the generators4 and 5 are in phase and have the same pulse width. The rest periods aresufficiently long to enable the polarization to fade away, as shown byFIG. 10, the measurements being taken during these rest periods andpreferably at the end, as shown by FIG. 10.

Referring to FIG. 11 through 13, the currents applied by the generators4 and S are still in phase but the pulse widths are different. The restperiods are still large enough to allow the polarization to fade away,and the measurements are preferably taken at the end of the restperiods.

The nature of the electrodes 1 and 2 must be compatible with thecomposition of the electrolyte 6 and with other electrodes which may beimmersed in this same electrolyte 6. The structure of the active part ofthe electrodes 1 and 2 must be such that the diffusion of theelectrolyte takes place within a minimum time. The size and positioningof the electrodes 1 and 2 must be chosen so that they are immersed in apart of the electrolyte which does not have an appreciable concentrationgradient, and that the composition of the electrolyte in that place isrepresentative of the average concentration.

For example, in the case of an element of a lead battery, theseelectrodes may be of the Plante" type and are preferably located at acertain distance from the upper and lower surfaces of the vesselcontaining the electrolyte.

The auxiliary electrode 16 may be placed in any part of the electrolytewhatever and may be neutral or capable of electrochemical exchanges. Theauxiliary electrode 16 may also be formed of one of the electrode of theelectrochemical generator itself, or even of a conductive coating of thevessel containing the electrolyte as illustrated schematically at 14 inFIG. 3.

Since the currents maintaining the measuring electrodes on charge arepermanently applied the electrodes do not require a large capacity. Itis therefore advantageous to employ electrodes of small active areawhich enables the obtention of very low time constants, the exchangebetween the active electrolyte and the ambient electrolyte being thenvery rapid. In order to obtain precise measurement, the conductors 8,9and 10 should not present troublesome contact or junction potentialdrops. With the device described above the concentration of theelectrolyte may be obtained with precision and reliability.

When the current supply circuits of each of the electrodes 1 and 2 areseparate, as in FIG. 3, it is impossible to compensate for possiblevariations in the measuring conditions, and more particularly thetemperatures conditions for each of the electrodes 1 and 2independently. This correction may be carried out automatically byappropriate conventional circuits regulating the current of eachgenerator. The use of electrodes with a low response time enables rapidvariations of concentration to be detected and measured. The devicesdescribed do not cause any deterioration or any influence upon othercircuits or elements connected to any electrode immersed in the sameelectrolyte.

A specific embodiment used to measure the electrolyte of submarine cellsuses two lead measuring electrodes having lateral surfaces equal to S 7cm and any auxiliary electrode (for example, of lead) whose surface isequal to S 3 cm.

In this case, currents are:

for the negative electrode 4 l ,uA

for the positive electrode 6].LA

For a concentration ranging from 3 percent to 38 percent, the electrodevoltage varies linearly from 1.86 V to 2.15 V.

The above description has been given merely by way of a non-limitingexample, and it is understood that modifications and variations may beintroduced without departing from the scope of the invention.

That which is claimed is:

1. A device for measuring the concentration of an electrolytecomprising: two measuring electrodes charged to an initial conditionsubmerged into the electrolyte, a current source means for supplyingpermanently to each electrode during the entire duration of themeasuring process a quantity of electricity adapted to compensate forany loss of charge and to thus maintain the electrode in its initialcharged condition, and voltage measuring means for measuring thepotential difference between the electrodes, said potential differencebeing indicative of the concentration of the electrolyte.

2. A device according to claim 1, in which said current source comprisesa current generator the terminals of which are connected respectively tothe said two measuring electrodes, said electrodes having activesurfaces the ratio of the areas of which is equal to the inverse of theratio of the quantities of current per surface unit necessaryrespectively to each of the electrodes in order to maintain them attheir initial state of charge.

3. A device according to claim 1 in which said current source comprisesfirst and second current generators and an auxiliary electrode, saidauxiliary electrode being adapted to be immersed in said electrolyte andbeing connected to one terminal of said first current generator, theother terminal of which is connected to one of the said measuringelectrodes, and said auxiliary electrode being further connected to oneof the terminals of said second current generator, the other terminal ofwhich is connected to the other measuring electrode, said currentgenerators being capable of delivering currents maintaining respectivelythe initial states of charge of said measuring electrodes.

4. A device according to claim 3 in which said current generators areconstant amplitude direct current generators.

5. A device according to claim 3, in which said first current generatoris a constant amplitude direct current generator and said secondgenerator is a pulse generator, the voltage measuring means connectedbetween two measuring electrodes being operable to carry out measurementwhen the current of said generator is equal to zero.

6. A device according to claim 5, in which said voltage measuring meansis operable to carry out measurements substantially at the end of a restperiod of said second current generator.

7. A device according to claim 3, in which said current generators areoperable to deliver substantially square-wave pulses, the pulses of saidsecond generator being synchronized and in phase with the pulses of saidfirst generator, said volt-' age measuring means being connected betweensaid measuring electrodes and being operable to carry out measurementduring the periods when the currents of said generators aresimultaneously equal to zero.

8. A device according to claim 6, in which the pulses produced by saidfirst generator are of different duration from the pulses produced bysaid second generator.

9. A device according to claim 3 in which the electrolyte, theconcentration of which is to be determined, is held in a vessel forminga part of an electrochemical generator, the auxiliary electrode beingformed by one of the electrodes of said electrochemical generator.

10. A device according to claim 3, in which the electrolyte, theconcentration of which is to be determined, is held in a vessel and theauxiliary electrode is formed by an electrically conducting coatingdeposited on at least a part of the inner surface of said vessel.

11. A device according to claim 10, in which said vessel is the casingof an electrochemical generator.

12. A device according to claim 3 wherein the electrolyte is held in avessel forming a part of an electro-chemical generator, the auxiliaryelectrode being formed by one of the electrodes of said electrochemicalgenerator.

13. A device according to claim 3, wherein said first and

1. A device for measuring the concentration of an electrolytecomprising: two measuring electrodes charged to an initial conditionsubmerged into the electrolyte, a current source means for supplyingpermanently to each electrode during the entire duration of themeasuring process a quantity of electricity adapted to compensate forany loss of charge and to thus maintain the electrode in its initialcharged condition, and voltage measuring means for measuring thepotential difference between the electrodes, said potential differencebeing indicative of the concentration of the electrolyte.
 2. A deviceaccording to claim 1, in which said current source comprises a currentgenerator the terminals of which are connected respectively to the saidtwo measuring electrodes, said electrodes having active surfaces theratio of the areas of which is equal to the inverse of the ratio of thequantities of current per surface unit necessary respectively to each ofthe electrodes in order to maintain them at their initial state ofcharge.
 3. A device according to claim 1 in which said current sourcecomprises first and second current generators and an auxiliaryelectrode, said auxiliary electrode being adapted to be immersed in saidelectrolyte and being connected to one terminal of said first currentgeneratoR, the other terminal of which is connected to one of the saidmeasuring electrodes, and said auxiliary electrode being furtherconnected to one of the terminals of said second current generator, theother terminal of which is connected to the other measuring electrode,said current generators being capable of delivering currents maintainingrespectively the initial states of charge of said measuring electrodes.4. A device according to claim 3 in which said current generators areconstant amplitude direct current generators.
 5. A device according toclaim 3, in which said first current generator is a constant amplitudedirect current generator and said second generator is a pulse generator,the voltage measuring means connected between two measuring electrodesbeing operable to carry out measurement when the current of saidgenerator is equal to zero.
 6. A device according to claim 5, in whichsaid voltage measuring means is operable to carry out measurementssubstantially at the end of a rest period of said second currentgenerator.
 7. A device according to claim 3, in which said currentgenerators are operable to deliver substantially square-wave pulses, thepulses of said second generator being synchronized and in phase with thepulses of said first generator, said voltage measuring means beingconnected between said measuring electrodes and being operable to carryout measurement during the periods when the currents of said generatorsare simultaneously equal to zero.
 8. A device according to claim 6, inwhich the pulses produced by said first generator are of differentduration from the pulses produced by said second generator.
 9. A deviceaccording to claim 3 in which the electrolyte, the concentration ofwhich is to be determined, is held in a vessel forming a part of anelectrochemical generator, the auxiliary electrode being formed by oneof the electrodes of said electrochemical generator.
 10. A deviceaccording to claim 3, in which the electrolyte, the concentration ofwhich is to be determined, is held in a vessel and the auxiliaryelectrode is formed by an electrically conducting coating deposited onat least a part of the inner surface of said vessel.
 11. A deviceaccording to claim 10, in which said vessel is the casing of anelectrochemical generator.
 12. A device according to claim 3 wherein theelectrolyte is held in a vessel forming a part of an electro-chemicalgenerator, the auxiliary electrode being formed by one of the electrodesof said electrochemical generator.
 13. A device according to claim 3,wherein said first and said second current generators are operable todeliver substantially square-wave pulsating currents, the pulses of thecurrent of said second generator being synchronized and in phase withbut having a different duration from the pulses of said first generator,said voltage measuring means connected between said measuring electrodesbeing operable to carry out measurement during the periods when thecurrents of said first and second generators are simultaneously equal tozero.